The present invention relates to a mass spectrometer and a method of mass spectrometry.
Wiley and McLaren (Time-of-Flight Mass Spectrometer with Improved Resolution, (Review of Scientific Instruments 26, 1150 (1955), W C Wiley, I H McLaren) set out the basic equations that describe two stage extraction Time of Flight mass spectrometers. The principles apply equally to continuous axial extraction Time of Flight mass analysers and orthogonal acceleration Time of Flight mass analysers and time lag focussing instruments.
FIG. 1 shows the principle of second order spatial (or space) focussing wherein ions with an initial spatial distribution are brought to a focus at the plane of an ion detector thereby improving instrumental resolution.
An ion beam with initial energy ΔVo and with no initial position deviation has a time of flight in the first acceleration stage Lp (called the “pusher” in an orthogonal acceleration Time of Flight instrument) given by:
                    t        =                              1            a                    ⁢                                                                      2                  ⁢                  q                                m                                      ·                          [                                                                    (                                          Vp                      ±                                              Δ                        ⁢                                                                                                  ⁢                        Vo                                                              )                                                        1                    /                    2                                                  ±                                  Δ                  ⁢                                                                          ⁢                                      Vo                                          1                      /                      2                                                                                  ]                                                          (        1        )            wherein ions of mass m and charge q are accelerated at a rate a through a potential Vp.
The initial velocity vo is related to the initial energy ΔVo by the relation:
                    vo        =                                            2.              ⁢              Δ              ⁢                                                          ⁢              Vo                        m                                              (        2        )            
The second term in the square brackets of Eqn. 1 is referred to as the “turnaround time” which is a major limiting aberration in Time of Flight instruments. The concept of turn around time is illustrated in FIG. 2. Ions that start at the same position but with equal and opposite velocities will have identical energies in the flight tube given by:K·E=qVacc+½mv2  (3)
However, the ions will be separated by a turnaround time Δt which is smaller for steeper acceleration fields i.e. Δt2<Δt1. This is often the major limiting aberration in Time of Flight instrument design and instrument designers go to great lengths to minimise this term.
The most common approach to minimising this aberration is to accelerate the ions as forcefully as possible i.e. the acceleration term a is made as large as possible by maximising the electric field i.e. the ratio Vp/Lp. This is normally achieved by making the pusher voltage Vp large and the acceleration stage length Lp short. However, this approach has a practical limit for a two stage geometry as the Wiley McLaren type spatial focussing solution leads to shorter physical instruments which will have very short flight times as shown in FIG. 3. Very short flight times would require ultra fast high bandwidth detection systems which are impracticable.
A known solution to this problem is to add a reflectron wherein the first position of spatial focus is re-imaged at the ion detector as shown in FIG. 4. This leads to longer practical flight time instruments which are capable of relatively high resolution.
In conventional reflectron Time of Flight instruments the reflectron may comprise either a single stage reflectron or a two stage reflectron whilst in both reflectron and non-reflectron Time of Flight instruments the extraction region usually comprises a two stage Wiley/McLaren source. Usually within these geometries the objective is to achieve perfect first or second order space focusing or to re-introduce a small first order term to further improve space focusing.
It is known that a small first order term may be arranged to compensate for linear pre-extraction velocity-position correlations obtained in various ion transfer configurations.
Despite known approaches to space focusing, the practical performance of known Time of Flight instruments is limited by space focusing characteristics. These limitations are most evident in the relationship between resolution and sensitivity.
It is desired to provide an improved Time of Flight mass spectrometer.